Field of the Invention
The present disclosure relates to an image forming apparatus.
Description of the Related Art
An electrophotographic image forming apparatus has a problem in terms of stability of image density (density variation) as compared with an inkjet image forming apparatus, an offset printing apparatus, etc. For example, there is a case where charge holding amount of toner changes during continuous output, resulting in changing developability and transferability of the toner. As a result, the image density changes.
For example, Japanese Patent Application Laid-open No. 2003-228201 discloses an image forming apparatus, in which a patch image for density correction is formed, the density of the image (toner density) is detected by a sensor, and image data which is being continuously output is changed based on the detection result. Also, in the apparatus, a gamma look-up table (gamma LUT) is used to control a development condition. The gamma look-up table is one-dimensional conversion table (gradation correction table) of the image data. It is a table for converting a signal value of the image data input to an output signal value for forming an image of an ideal gradation characteristic by the image forming apparatus. However, with the gamma LUT, it is not possible to increase the maximum density of the image formed by the image forming apparatus.
On the other hand, US Patent Application Publication No. 2007/071471 discloses an image forming apparatus which controls to form a latent image by changing an exposure condition and to increase the toner amount to be developed to correct maximum density. It is noted that when changing the exposure condition (laser power) for correcting the maximum density, halftone density changes accordingly. Thereby, when changing the exposure condition, it is necessary to check the halftone density under the exposure condition. Also, the gamma look-up table needs to be corrected.
FIG. 14 represents laser power when the image forming apparatus forms the image on A3 size sheet (18 sheets). As shown in FIG. 14, in the image forming apparatus, the exposure condition is not changed so that the laser power is maintained at 50. FIG. 15 shows a conventional example of correcting the laser power based on a measurement result of an image for measurement. The image for measurement is formed in an area between a plurality of sheets (an area between a preceding sheet and a following sheet). FIG. 16 also shows a conventional example of correcting the laser power based on the measurement result of the image for measurement. In FIG. 16, by increasing an interval where a plurality of sheets are conveyed, the area between a plurality of sheets is widened. Thereby, the image forming apparatus can form a plurality of images for measurement in one area. It is noted that, in FIGS. 15 and 16, the image forming apparatus can execute two types of halftone processing, i.e. first pseudo halftone processing and second pseudo halftone processing.
As shown in a table in FIG. 15, based on a detection result of the image for measurement formed between a 1st sheet and a 4th sheet, the image forming apparatus determines the exposure condition of the image for measurement which is formed between an 11th sheet and a 14th sheet. It means that, when forming the image for measurement which corresponds to the first pseudo halftone processing, the image forming apparatus in FIG. 15 updates the exposure condition based on the detection result of the image for measurement which corresponds to the first pseudo halftone processing which is previously performed. This is because it is required to form the image for measurement based on the exposure condition to which the maximum density is guaranteed. Otherwise, it is not possible to properly correct both the maximum density and the halftone density. Further, the image forming apparatus in FIG. 15 updates the exposure condition based on the detection result of the image for measurement which corresponds to the same halftone processing. This enables to obtain the exposure condition for guaranteeing the maximum density with high accuracy. However, there is a possibility that the image forming apparatus shown in FIG. 15 cannot follow the density variation caused in a short time. Also, as shown in a table in FIG. 16, the image forming apparatus stops the image formation on the sheet, which causes downtime. Thereby, it is desired to provide an image forming apparatus which can suppress the downtime while suppressing the density variation.